*
 * <p>In order to allow proper garbage collection of unused locks, the edges of the lock graph are
 * weak references.
 *
 * <p><strong>Performance</strong>
 *
 * <p>The extra bookkeeping done by cycle detecting locks comes at some cost to performance.
 * Benchmarks (as of December 2011) show that:
 *
 * <ul>
 *   <li>for an unnested {@code lock()} and {@code unlock()}, a cycle detecting lock takes 38ns as

</div>

Das bedeutet, dass es **immer einen Wert** hat, der Wert kann jedoch manchmal `None` sein (oder `null` in JSON).

Das bedeutet, dass Clients, die Ihre API verwenden, nicht prüfen müssen, ob der Wert vorhanden ist oder nicht. Sie können davon ausgehen, dass das Feld immer vorhanden ist. In einigen Fällen hat es jedoch nur den Defaultwert `None`.

@item_router.get("/")
def get_items(user_id: Optional[str] = None):
    if user_id is None:
        return [{"item_id": "i1", "user_id": "u1"}, {"item_id": "i2", "user_id": "u2"}]
    else:
        return [{"item_id": "i2", "user_id": user_id}]


@item_router.get("/{item_id}")
def get_item(item_id: str, user_id: Optional[str] = None):
    if user_id is None:
        return {"item_id": item_id}
    else:

/**
 * A {@link FutureTask} that also implements the {@link ListenableFuture} interface. Unlike {@code
 * FutureTask}, {@code ListenableFutureTask} does not provide an overrideable {@link
 * FutureTask#done() done()} method. For similar functionality, call {@link #addListener}.
 *
 * <p>Few users should use this class. It is intended primarily for those who are implementing an

    private final T value;
    private boolean done;

    SingletonIterator(T value) {
      this.value = value;
    }

    @Override
    public boolean hasNext() {
      return !done;
    }

    @Override
    @ParametricNullness
    public T next() {
      if (done) {
        throw new NoSuchElementException();
      }
      done = true;
      return value;
    }
  }

## Parâmetros opcionais { #optional-parameters }

Da mesma forma, você pode declarar parâmetros de consulta opcionais, definindo o valor padrão para `None`:

{* ../../docs_src/query_params/tutorial002_py310.py hl[7] *}

Nesse caso, o parâmetro da função `q` será opcional, e `None` será o padrão.

/// check | Verifique

## Parámetros opcionales { #optional-parameters }

De la misma manera, puedes declarar parámetros de query opcionales, estableciendo su valor por defecto en `None`:

{* ../../docs_src/query_params/tutorial002_py310.py hl[7] *}

En este caso, el parámetro de función `q` será opcional y será `None` por defecto.

/// check | Revisa

    client = test_module.client
    response = client.get("/users/")
    assert response.status_code == 200, response.text
    assert response.json() == {
        "message": "Hello Users!",
        "params": {"q": None, "skip": 5, "limit": 10},
    }


def test_override_in_users_with_q(test_module: ModuleType):
    client = test_module.client
    response = client.get("/users/?q=foo")

        c.cookies.set("session_id", "123")
        response = c.get("/items/")
    assert response.status_code == 200
    assert response.json() == {
        "session_id": "123",
        "fatebook_tracker": None,
        "googall_tracker": None,
    }


def test_cookie_param_model_invalid(client: TestClient):
    response = client.get("/items/")
    assert response.status_code == 422
    assert response.json() == {
        "detail": [

app = FastAPI()


class Item(BaseModel):
    name: str
    price: float
    is_offer: Union[bool, None] = None


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}


@app.put("/items/{item_id}")
def update_item(item_id: int, item: Item):